(1) Field of the Invention
This invention relates to a refractive index variability measuring system and more particularly to a specific configuration which provides an improved method for active structural vibration and thermal expansion cancellation.
(2) Description of the Prior Art
This invention pertains in general to instruments for measurement of refractive index variability in natural waters. Such measurement is necessitated by the existence of refractive index inhomogeneities, or cells of refractive index variability, in the ocean and the like. These cells create acoustical interference phenomena when penetrated by acoustic waves as pointed out by Robert J. Urick in his book: Principles of Underwater Sound, 3rd Edition, McGraw-Hill Book Company, 1983 pp. 183-187. Measurements of these cells have applications involving performance of underwater optical systems which rely on the detection of phase changes in communication and imaging systems. General principles for the measurement of refractive index variability for both air and water are known in the field and have heretofore been described in the prior art such as Gibson et al.: "Optical Path Length Fluctuations in the Atmosphere", Appl. Opt., 23,4383-4389 1984; and Buchave et al: "The Measurement of Turbulence with the Laser Doppler Anemometer", Ann. Rev. Fluid Mech., 11,443-503, 1984. The measurement of refractive index fluctuations in air typically involves reflection of two sets of optical beams from plane parallel mirrors, with one set of beams passing through vacuum (thermal expansion and vibration canceling path) and the other set of beams passing through a medium such as air (measurement path). Instruments performing these measurements, are available commercially. For example, the Hewlett-Packard Wavelength Tracker HP1017A, and the measurements of changes in refractive index (dn) attained are described in Zygo Corporation, (Middlefield, Conn. 06455), Application Bulletin, of 22 Dec. 1987, "Optical Wavelength Compensator Description, Installation, and Alignment". Typical applications would be to use such an instrument to make corrections to displacement, velocity or angle measurements caused by fluctuations in the refractive index in the medium.
However, neither the instrumentation nor the methodology for atmospheric measurement of refractive index variability is directly transferable or can be easily modified for use in the ocean. To date differences in size of cells of refractive index variability and diffusion characteristics as well as technical deployment difficulties have hampered attempts to translate instrumentation and methods from the air to a medium such as water. An indirect method presently used is described in "Tow Chain Measurements of Ocean Microstructure", by Stephen A. Mack, J. Phys. Oceanogra., 19,1108-1129, 1989. This method involves chains of microtemperature and microconductivity probes towed through the seawater by a ship. However, indirect methods use approximate equations of state to obtain the refractive index variability leading to inaccuracies, proposals have been made to attempt direct measurements with interferometers that measure optical phase. One such attempt is by Richard C. Honey, "A proposed program to monitor the fine structure and microstructure of the upper layers of the sea", Final Report, Stanford Research Institute Project 1189, Nov. 30, 1971. However, the self noise of the optical devices themselves can create phase changes as well as refractive index variations, with the result that the structural vibrations may dominate the measurements. There is thus a need for canceling the effect of structural vibrations and thermal expansion on measurements of refractive index variability of the medium such as ocean water.